[hw-8] add: repo layer

comments/db/migrations/00001_create_comments.sql

@@ -0,0 +1,10 @@
+CREATE TABLE comments (
+  id          BIGSERIAL PRIMARY KEY,
+  user_id     BIGINT NOT NULL,
+  sku         BIGINT NOT NULL,
+  text        VARCHAR(255) NOT NULL,
+  created_at  TIMESTAMP(3) NOT NULL DEFAULT now()
+);
+
+CREATE INDEX ON comments(sku, created_at DESC, user_id ASC);
+CREATE INDEX ON comments(user_id, created_at DESC);